Method of detecting particles by detecting a variation in scattered radiation

ABSTRACT

A smoke detecting method which uses a beam of radiation such as a laser ( 16 ), to monitor a region, such as a room ( 12 ). A camera ( 14 ) is used to capture images of part of the room ( 12 ), including a path of the laser beam. Particles in the laser beam scatter light ( 30 ), and this is captured by the camera ( 14 ) for analysis. A processor ( 20 ) extracts data relating to the scattered light ( 30 ) to determine the density of particles in the beam, to determine the level of smoke in the region. The laser may have a modulated output ( 38 ) so that images captured without the laser tuned “on” can be used as a reference point and compared to images taken with the laser turned “on”, to assist in determining the level of scattered light ( 30 ) compared to ambient light. Filters ( 24, 26 ) may be used to decrease signals generated from background light.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a Continuation application of U.S. application Ser. No.14/089,315, filed Nov. 25, 2013, which is a Continuation application ofU.S. application Ser. No. 13/775,577, filed Feb. 25, 2013, now U.S. Pat.No. 8,620,031 issued Dec. 31, 2013, which is a Continuation applicationof U.S. application Ser. No. 13/164,123, filed Jun. 20, 2011, now U.S.Pat. No. 8,406,471 issued Mar. 26, 2013, which is a Continuationapplication of U.S. application Ser. No. 10/556,807, filed Nov. 9, 2006,now U.S. Pat. No. 7,983,445 issued Jul. 19, 2011, which is a U.S.National Stage Application of PCT/AU2004/000637 filed May 14, 2004,which claims priority to Australian Provisional Patent Application No.2003902319, filed May 14, 2003 and entitled “Laser Video Detector”. Theabove-noted applications are incorporated herein by reference in theirentirety.

FIELD OF THE INVENTION

The present invention relates to an improved sensor apparatus andimproved method of sensing. In particular the present invention relatesto an improved particle 10 detector and method of detecting particles.

BACKGROUND OF THE INVENTION

There are a number of ways of detecting smoke in a region, such as aroom, building, enclosure, or open space. Some methods involve samplingair from the region and passing the sampled air through a detectionchamber, whereby particles are detected and an estimation is made of theamount of smoke in the region of interest. Such an apparatus isexemplified in aspirated smoke detectors like LaserPLUS™ sold by theapplicant. Other detectors are placed in the region of interest, and usea sensor to detect particles adjacent the sensor. An example of such adetector is a point detector, in which air passes between an emitter anda sensor, and the smoke is detected directly in the region of interest.

In both cases if the smoke does not enter a sampling point (of theaspirated detector) or pass between the sensor and emitter of the pointdetector, no smoke will be detected. As many buildings employ airhandling means for extracting air from a region, such asair-conditioning, there is no guarantee that smoke will be detectedrather than pass out of the region via the air handling ducts. It can bevery difficult to use the aforementioned methods of detecting smoke inoutdoor areas or very large indoor arenas where there may not beappropriate locations to place a point detector or a sample point andconnecting tubing.

Other devices used to detect smoke include the detector disclosed inU.S. Pat. No. 3,924,252, (Duston) which uses a laser and a photodiode todetect light scattered from particles. This device uses a cornerreflector to reflect the light back at the emitter. Duston requires afeedback circuit to detect whether the beam is emitted or blocked.

Another type of detector is known as a “Beam Detector”, which measuresthe attenuation of the intensity of a signal from a projected lightsource caused by smoke particles suspended in the projected light. Thesedetectors have relatively low sensitivity and are only capable ofmeasuring the total attenuation within the illuminated region.

Any discussion of documents, devices, acts or knowledge in thisspecification is included to explain the context of the invention. Itshould not be taken as an admission that any of the material forms apart of the prior art base or the common general knowledge in therelevant art in Australia or elsewhere on or before the priority date ofthe disclosure and claims herein.

SUMMARY OF THE INVENTION

In one form the present invention provides a method of detectingparticles including emitting a beam of radiation into a monitored regionand detecting a variation in images of the region indicating thepresence of the particles.

With respect to the above method, further steps embodying the method andfeatures of preferred embodiments may include identifying an area ofinterest in the images which represents a corresponding zone of themonitored region. Scattered radiation within the zone may be representedin one or more segments of a corresponding image, which allows for thelocation of the particles in the region to be identified. The locationof the particles may be determined in accordance with a geometricrelationship between the locations of a source of emitted radiation, adirection of the emitted radiation and a point of image detectionwherein, the geometric relationship is determined from the images. Thedetected variation may be an increase in scattered radiation intensity.The increase in scattered radiation intensity may be assessed withreference to a threshold value. The threshold value may be calculated byaveraging integrated intensity values from the images. The method maycomprise assigning different threshold values for different spatialpositions within the region. The method may comprise directing theradiation along a path and identifying a target in the images, thetarget representing a position at which the radiation is incident on anobjective surface within the region. A location of the target in theimages may be monitored and the emission of radiation may be ceased inresponse to a change in the location of the target. The method compriseidentifying a location of an emitter in the images. Further, the methodmay comprise determining an operating condition of the emitter based onradiation intensity at the identified location of the emitter. Theimages may be processed as frames which are divided into sections whichrepresent spatial positions within the monitored region. Also, themethod may comprise monitoring intensity levels in associated sectionsof the images and assigning different threshold values for differentspatial positions within the region which correspond to the associatedsections.

In another aspect, the present invention provides apparatus formonitoring a region, comprising:

an emitter for directing a beam of radiation comprising at least onepredetermined characteristic into the region;

an image capture device for obtaining at least one image of the region;and

a processor for analysing the at least one image to detect variation ofthe at least one characteristic between the images, indicating presenceof particles within the region.

The processor may be adapted to determine the location of particles inaccordance with a geometric relationship between the locations of theemitter, the directed beam of radiation and the image capture devicewherein, the geometric relationship is determined from the analysedimages. The apparatus may comprise a plurality of emitters, arranged todirect radiation along different respective beam paths. The apparatusmay further comprise one or more filters for adapting the image capturedevice to capture radiation from the emitter in preference to radiationfrom other sources. The filters may be one or more or a combination of:

a temporal filter.

a spatial filter.

a band-pass filter.

a polarising filter.

The image capture device preferably comprises an attenuator. Theattenuator may comprise a variable aperture device. A plurality ofimage-capturing devices may be used. Preferably, the image capturedevice comprises a camera. It is also preferable that the emittercomprises a laser.

In a further aspect, the resent invention provides a method of detectingparticles comprising the steps of: determining a path of a beam ofradiation comprising placing a first image capturing device to view asource of the radiation and at least a part of the path of the beam ofradiation; communicating the position of the source to a processor;placing a second image capturing device to view an impact point of thebeam of radiation; communicating related position information of theimpact point to the processor; determining the path of the beam inaccordance with a geometric relationship between the position of thesource and the position information of the impact point.

In yet another aspect the present invention provides a method ofdetecting particles comprising the steps of determining a region ofinterest containing a path of a beam of radiation comprising locating afirst point, being the position of a source of the beam, using an imagecapturing device; locating a second point being the intersection of thebeam of radiation with a field of view of the image capturing device,determining the path of the beam in accordance with the first and secondpoint; calculating a region of interest containing the determined beampath.

The step of locating a second point may be performed with at least onesubstantially transparent probe and the probe is preferably removed fromthe beam path once located.

In still another aspect, the present invention provides a method ofdetermining the level of smoke at one or more subregions in a region ofinterest comprising: directing a beam of radiation within the region,selecting a view of at least a portion of a path of the beam with animage capture device, determining the location of the source of theradiation relative to the image capture device, determining thedirection of the beam relative to the image capture device, dividing thebeam of radiation into segments, determining a geometric relationshipbetween the segments and the image capture device, adjusting a level oflight received by the image capture device of each segment so as toallow for the geometric relationship. The segments may comprise at leastone pixel and the segments are preferably grouped to form the subregionsfor smoke detection.

In a further aspect the present invention provides apparatus adapted todetect particles, said apparatus comprising processor means adapted tooperate in accordance with a predetermined instruction set, saidapparatus, in conjunction with said instruction set, being adapted toperform the method as disclosed herein.

In embodiments of the present invention there is provided a computerprogram product comprising; a computer usable medium having computerreadable program code and computer readable system code embodied on saidmedium for detecting particles within a data processing system, saidcomputer program product comprising; computer readable code within saidcomputer usable medium for performing the method steps the methods asdescribed herein.

Other aspects, advantages and features are disclosed in thespecification and/or defined in the appended claims, forming a part ofthe description of the invention.

Further scope of applicability of the present invention will becomeapparent from the detailed description given hereinafter. However, itshould be understood that the detailed description and specificexamples, while indicating preferred embodiments of the invention, aregiven by way of illustration only, since various changes andmodifications within the spirit and scope of the invention will becomeapparent to those skilled in the art from this detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

Further disclosure, improvements, advantages, features and aspects ofthe present application may be better understood by those skilled in therelevant art by reference to the following description of preferredembodiments taken in conjunction with the accompanying drawings, whichare given by way of illustration only, and thus are not limiting to thescope of the present invention, and in which:

FIG. 1 shows a schematic representation of an embodiment of a detectorsystem 25 from a side view;

FIG. 2 shows a top plan view of an embodiment of an image capture deviceand emitter position of the detector system of FIG. 1;

FIG. 3 shows a schematic perspective representation of an image taken byan image capture device of FIG. 2;

FIG. 4 shows a system overview workflow for signal processing for thedetector system of FIG. 1;

FIG. 5 shows a graphical representation of segmentation of data capturedby the image capture device in the embodiment of FIG. 1;

FIG. 6 shows a graphical representation of the integration of the datacaptured by the image capture device of the embodiment of FIG. 1;

FIG. 7a-c shows images illustrating background cancellation performed bythe detection system of FIG. 1;

FIG. 8 shows a graphical representation of a method used for calculatingpixel radius in an embodiment of the software used in conjunction withthe operation of the detector system of FIG. 1;

FIG. 9 is a top plan schematic view of a second embodiment of a detectorsystem in accordance with the present invention;

FIG. 10 is a top plan schematic view of a third embodiment of a detectorsystem in accordance with the present invention;

FIGS. 11a-c are top plan schematic views of fourth, fifth and sixthembodiments of the detector system in accordance with the presentinvention;

FIG. 12 shows a schematic representation of a part of the detectorsystem of FIG. 1;

FIG. 13 shows a schematic representation of captured image data from animage capture device of the detector system of FIG. 1;

DETAILED DESCRIPTION OF THE INVENTION

In FIG. 1, an embodiment of a particle detector 10 is shown. Thedetector 10 is located in a region 12 to be monitored. The region couldbe a room, stadium, hallway, or other area. It is not necessary for theregion to be enclosed or indoors.

An image capture device 14 views at least a portion of the region 12,comprising a portion that contains electromagnetic radiation fromemitter 16. The image capture device 14 may be a camera or one or moredevices forming a directionally sensitive electromagnetic receiver suchas photodiodes or CCD's, for example. In the preferred embodiment, theimage capture device 14 is a camera. In the present embodiment, thecamera 14 uses full frame capture to capture the images to send analoguevideo information along communications link 18 to a processor 20. It isnot necessary to use full frame capture. However, it is preferable touse full frame capture for engineering simplicity in obtaining images,performance, and minimising installation restrictions. As would beunderstood by the person skilled in the art, other image capture devices14 such as line transfer cameras may be used and methods to compensatefor the efficiency of full frame capture may be employed. Anothercommunication link 22 connects the emitter 16 to the processor 20. Theprocessor 20 controls the output of emitter 16, and/or receivesinformation about the output of emitter 16 through the communicationslink 22. Alternatively, the state of the emitter 16 may be sensed by thecamera 14 or determined automatically as disclosed below. In thepreferred embodiment, the emitter 16 is a laser producing visible,infra-red or other suitable radiation. The laser 16 may incorporate alens 21 and spatial filter such as a field of view restrictor 23. When abeam of light travels thought a homogeneous medium there is noscattering, only when irregularities are present does the beam scatter.Therefore, in the presence of particles such as smoke particles thelaser beam will scatter. Furthermore, in accordance with the preferredembodiment, the laser 16 may be modulated, eg “laser on”, laser “off” ina given sequence. When no smoke is present, the intensity of pixels in acaptured image including the laser beam is the same regardless of thestate of the laser. When smoke is present, there is a difference betweenthe intensity of a captured image when the laser 16 is on (due toscattering), compared to the intensity when the laser 16 is turned off.

Optional filters are shown in FIG. 1 in the form of a polarizing filter24 and a band pass filter 26. The polarising filter 24 is adapted toallow electromagnetic radiation emitted from the emitter 16 to passthrough, while preventing some of the background light from entering thecamera 14. This is useful if the emitter 16 is a laser emittingpolarised light, then the polarising filter 24 can be aligned with thepolarisation angle of the laser beam to allow maximum transmission oflaser light, while removing some background light, which typically isfrom randomly or non polarised light sources. The second filter 26 is aband pass filter, which attempts to only allow light within apredetermined frequency range (i.e. the frequency of the electromagneticradiation from the emitter 16). For example, an interference filter orcoloured gel may be used as the band pass filter 26. By using a bandpass filter (for example allowing substantially only light around 640 nmif a red laser of that frequency is used), significant background lightwill be removed, increasing the relative intensity of light scatteredfrom particles suspended in the air in the region 12.

Other filtering methods comprise modulation of the laser and use ofpositional information with regard to the systems components asdescribed below.

The image capture device may employ an attenuator for controlling theradiation received. A controllable neutral density filter arrangementmay be used. Alternatively, the attenuator could be in the form ofcontrolling the intensity with a variable aperture. An optional,adjustable, iris 24 a can be used to control exposure levels. It can bemanually set at the time of installation, or the system couldautomatically set the exposure according to incident light levels. Thereason for this is to minimise or avoid camera saturation, at least inthe parts of the field of view that are used in subsequent processing.The iris 24 a could be a mechanical iris or an LCD iris or any othermeans to reduce the amount of light entering the camera. Some electroniccameras incorporate an electronic shutter, and in this case the shuttertime can be used to control exposure instead of an iris 24 a. A spatialfilter 24 b is also shown, which may for example comprise a slit foreffectively masking the incident light to the camera 14. For example, aslit may mask the incident received light at the camera 14 to conformgenerally to the shape of the laser beam as it would be projected in theplane of the camera 14 lens. Items 26, 24 a, 24 b & 24 can be physicallylocated in a variety of orders or combinations.

In use, electromagnetic radiation, such as a red laser light fromemitter 16, passes through the region 12 and impacts on a wall or anabsorber 28. The field of view of the camera 14 comprises at least partof the path of the laser, and optionally, the impact point of the laseron the wall, which in this case impacts on an absorber 28. Particles inthe air in the region that intersect the laser, in this case representedby particle cloud 30, will cause laser light to scatter. Some of thelight scattered from particles will fall on the sensor of the camera 14,and be detected.

In the embodiment shown in FIG. 1 the camera 14 outputs analogueinformation to a video capture card 32 of the processor 20. The videocapture card 32 converts the analogue information to digital informationwhich is then further processed by computer 34. The processing isundertaken by software 36 running on the computer 34, which will bedescribed later. In the preferred embodiment, the processing is carriedout in order to interpret the captured image(s) such that an image planecorresponds to or is mapped to corresponding positions on the laserbeam. This may be achieved by relatively straightforward geometry andtrigonometry once predetermined location or position information of thesystem's components is obtained.

In other. embodiments it is possible to use a camera 14 which wouldcapture the data and transmit it digitally to the processor 20 withoutthe need for a video capture card 32. Further, the camera 14, filters24, 26, processor 20 and light source 16 could be integrated into asingle unit or units. Also, embedded systems may be employed to providethe functions of at least the processor 20.

A number of camera 14 configurations are able to be used in thisapplication, provided image information in the form of data can besupplied to the processor 20.

In the example shown in FIG. 1, a laser modulator 38 is used to vary thepower of the emitter 16. The power level can be changed to suit lightingconditions, meet eye safety requirements and provide on/off modulation.In this embodiment, the camera 14 captures 30 frames every second, theemitter 16 is cycled on for one frame and off for the next. The amountof light in a region is sensed for each frame, and the sum of the lightin a region when the laser is off is subtracted from the sum of lightreceived while the laser is on. The sums may be over several frames. Thedifference between the sum of light received when the laser is oncompared to the light received when the laser is off is taken as ameasure of the amount of scattering in that region. To act as an alarm,a threshold difference is set and should the difference be exceeded, thealarm may be activated. In this way the detector 10 may act as aparticle detector. As measuring the scattered light from particles isknown to be a method of determining whether there is smoke in a region,the detector 10 may be used as a smoke detector. More detail on thecancellation, filtering and software is provided below.

The detector 10 may be set to wait until the measured scattering exceedsa given threshold for a predetermined period of time, before indicatingan alarm or pre-alarm condition. The manner for determining an alarm orpre-alarm condition for the detector 10 may be similar to the methodsused in aspirated smoke detectors using a laser in a chamber, such asthe VESDA™ LaserPLUS™ smoke detector sold by Vision Fire and SecurityPty Ltd.

FIG. 2 shows a top view of the embodiment in FIG. 1. The camera 14 has afield of view 8, which in this case covers substantially all the region12, which may be a room in a building. The light from emitter 16 isdirected generally towards the camera 14, but not directly at the lens.There is therefore an angle subtended by an imaginary line between thecamera 14 and the emitter 16, and the direction of the laser beam. Theangle may be in the horizontal plane as shown by angle z in FIG. 2,and/or the vertical plane as shown by angle x. in FIG. 1. The laser beamdoes not impact on the camera lens directly. Nonetheless, the laser beampath will be in the field of view of the camera 14, as shown in FIG. 3.

Physical System Variations

It is desirable in some circumstances to use a number of emitters in asystem. This may be to comply with regulations, provide back up, or toassist in covering a larger area than could be covered with a singleemitter.

If coverage of a large area is required, it is possible to employ anumber of emitters so that smoke may be detected in a number ofdifferent locations within a region. FIG. 9 shows an arrangement wherebycamera 50 is located within a region such a room 52. If detection wasrequired across a large area, multiple lasers 54 and 55 could be spreadaround the room to provide coverage. FIG. 9 shows the emitters groupedinto two groups, with emitters from group 54 targeted at point 56 andemitters 55 targeted at point 57. The camera 50 may have the points 56and 57 in view, or may not see the points 56 and 57. Camera 50 may havepoints 56 and 57 in view by way of an optical arrangement to project animage of points 56 and 57 into the field of view of camera 50, forexample, rear view minors (not shown) placed forward of camera 50.Likewise a prism or some other optical system could achieve this result.Further, the emitters 54 and 55 may all be on simultaneously, or may becycled, so that if the camera 50 can detect the point at which theradiation lands, the radiation detected in the camera can be used toverify that the emitter is operating and not blocked. Detection ofindividual emitters is possible if they were switched on and offsequentially, or in any sequence of patterns that are not linearlydependant, so that using timing information, it is possible to detectwhich emitter is on at any one time. Further, knowing which emitter wasfiring would allow the detector to localise sub regions in the area tobe protected and ascertain where any detected particles were locatedwith respect to the sub regions. In effect the beam or beams that havebeen scattered by particles may be determined.

The emitters 54 and 55 do not all need to intersect on targets 56 and57, and may be distributed along a number of targets, or cross over eachother onto other targets.

An alternative is shown in FIG. 10, where the lasers 58 and 59 are aimedaway from the camera 60. The camera 60 can detect a light from the laserlight hitting the wall at point 61 and 62. If either of these pointsdisappears, then the detector system knows that either a laser is faultyor something is blocking the path of the laser light. If the laser isblocked, generally the object blocking the laser light will also reflectthe light, and therefore the laser spot will shift from the mown targetarea, that is original point 61 or 62. The camera can detect the shiftin the spot and may sound an alarm or turn the laser off. This may beimportant, especially if the laser is not considered eye safe. Anothermeans by which faults may be detected is when a spurious object such asa spider web intersects with a beam causing scattering. Occasionalmovement of the emitted beam, for example by translating the emitter ina lateral direction, will obviate such false detections of scatteredradiation.

In FIG. 10 a second camera 63 is shown which may be connected to thesystem to provide additional views. Using two cameras may allow a moreaccurate means of locating the area of smoke than using a single camera.Also, the additional view will provide scattering information fordifferent scattering angles for the same particulate material. This datacan be used to discriminate between materials with different particlesize distributions or scattering properties. This in turn can be used toreduce the system sensitivity to nuisance particles that might otherwisecause false alarms such as dust, for example. With the use of one ormore emitters, variation in scattering angle; wavelength of emittedradiation; polarisation rotation; plane of polarisation of viewedscattering and varying the timing of emission and detection all providemeans for discriminating between different types of particles.

In FIG. 11a camera 64 views two lasers 65 and 66 that cross the room.FIG. 11b uses a laser that is reflected back towards the camera 67, toprovide better room 30 coverage and capture both forward and backwardscattered light.

In the present embodiment, the processor 10 comprises a personalcomputer running a Pentium 4 chip, Windows 2000 operating system.

An important aspect of the present embodiments is signal processing isdiscussed in detail below with reference to FIG. 4 which is a data flowdiagram, the layout of which, would be understood by the person skilledin the art. For ease of reference, the signal processing in thisembodiment is conducted using software for the detector 10, referred toas LVSD software. It is to be noted with reference to FIG. 4 that thedata flow lines indicate image data flow, array data flow and simplenumeric or structured data flow at different stages of the processing.Thus, some of the processing functions described may handle the moreintensive image data or optionally, the less intensive numeric data, forexample. As would be understood by the person skilled in the art,engineering efficiencies may be attained by choice of the components andsoftware entities used to carry out the processing functions at theserespective stages.

Laser State Determination

At step 401 of FIG. 4 a determination of the laser state is performed.The LVSD software in this embodiment relies on having the laser sourcewithin the field of view of the camera in order to determine the stateof the laser for a particular frame.

A small region of interest is assigned that includes the laser sourceradiation. The centre of the region is set to an initial position of thelaser source spot. The average pixel value in the region is computed. Itis then compared with a threshold value to make the decision of whetherthe image records the laser on or off.

The threshold value is the average of the outputs of a peak detector anda trough detector that are fed by the average. Each detector executes anexponential decay back to the current average in the case that a newpeak or trough has not been made. The time constant is set in terms offrames, preferably with values of about 10.

This technique has proven to be fairly robust. An alternative method isto look for one or more pixels that exceeded the average in therectangle by a fixed threshold.

In an implementation where the laser on/off switching is more closelycoupled to frame acquisition this function may not be required. However,it can still serve a double check that the laser source is not obscuredand is of the correct intensity.

Laser Position

At step 401 of FIG. 4, a centre of gravity algorithm estimates the pixelcoordinates of the laser source within the area being monitored. Thispositional information is optionally updated at every “laser on” imageto allow for drift in either the laser source or camera location due tomovement of the mounts and/or building over time. The factors affectingthe stability comprise movement of walls within the building, mountingpoint rigidity etc.

More precisely, the threshold established in the previous step (laserstate determination) is subtracted from the image and negatives areclipped to zero. The centre of gravity of the same rectangle used in thestate determination then yields (x,y) coordinates of the laser spot. Inthis calculation, the pixel values are treated as weight.

An alternative technique is to treat the previously described area as animage and calculate an average of a large number (−50) of known “emitteroff state” images, then subtract the average from the latest image thatis known to have been captured with the emitter on. The previouslydescribed centre of gravity algorithm is then applied to the image datato estimate the position of the spot.

Compute Regions of Interest & Background Cancellation

At step 403 of FIG. 4, regions of interest are calculated. At step 404of FIG. 4, background cancellation is performed. A combination ofinterpolation and frame subtraction is used during backgroundcancellation to reduce interfering temporally variant and invariantinformation from the image. The image is segmented into three regions ofinterest as shown in FIG. 5. The background is segmented into backgroundregions 101 and 103, and there is an integration region 102. Theseregions are updated periodically to reflect any detected changes in thelaser source location. The choice of shape of the regions of interestreflects the uncertainty in the precise position in the image of thescattered radiation. in FIG. 5 the camera cannot see the point where theemitted radiation hits the wall, and therefore the exact path of theemitted radiation is unknown. This produces a region of interest 102that expands as the distance from the emitter increases. A method ofdetermining the path of the emitted radiation manually is to test thelocation of the emitted radiation by blocking the radiation temporarilyand checking its position, then entering the data manually into theprocessor. Alternatively, one or more substantially transparent probes,which may be in the form of articles such as plates, may be insertedinto the beam. Some scattering will occur on entry and exit from theplate providing a reference point or points in the image from which therequired integration area and background areas may be computed. Inapplications where the detector may be used for detecting particles in,for example, clean room or hazardous environments, the windows of suchenclosures may act as the substantially transparent plates and, thesetherefore may establish the path of the beam without the need to intrudeinto the environments to install the detector system components. Thepurpose of a narrow integration area is to reduce the noisecontributions from pixels that are not contributing a scattering signaland also to allow the background regions to be closer to the integrationregion thus allowing a better estimate of the correction factor that isused for correcting the illumination level in the laser off images.

The integration region 102 contains the emitted radiation path, whilethe areas to each side, background region 101 and 103, are used duringbackground cancellation. The regions are generally triangular, that iswider further away from the laser source. This is necessary becausewhile the exact location of the radiation spot is known, the exact angleof the path is not, so a greater tolerance is needed at the other end ofthe path when the camera cannot see where the radiation terminates.There is more noise in a fatter section of integration region due tomore pixels, fortunately, each pixel represents a shorter length of thepath, so the larger number of samples per unit length allows moreaveraging. If the camera can see the radiation termination point, therewould be less uncertainty of its position and the regions of interestwould not need to diverge as much as shown in FIG. 5.

Two background regions 101, 103 are chosen for interpolation of thebrightness compensation factor for correcting temporal variations inbackground lighting on either side of the radiation path in the laseroff images. For example, changes in lighting due to two different,independent temporally varying light sources on either side of theradiation path. This principle could be further extended to allow forvariations along the path, not just to either side of the path bysubdividing the three areas 101, 102,103 into segments along the lengthof the radiation path and performing the calculations for eachsubdivision.

The background cancelling algorithm sums n “on frames” and m “offframes” the sequence of these frames is arbitrary. Prior to thesubtraction of the “emitter off” frames from the “emitter on” frames,the “emitter off” frames are scaled by a factor, f, to compensate forvariance in lumination levels of the images. This may be useful withartificial lighting, the intensity of which varies rapidly. Theresultant image contains any differences between the n “emitter on” andM “emitter off” images. This is shown graphically in FIG. 6.

The scaling factor f is determined by interpolation, using the ratios ofbackground variation between the laser on and laser off frames.

$f = \frac{\left( {\frac{\mu_{{on}\; 1}}{\mu_{{off}\; 1}} + \frac{\mu_{{on}\; 2}}{\mu_{{off}\; 2}}} \right)}{2}$

where:

μ is the average value of pixel intensity in a given background regionin either a laser on or laser off frame as designated by the subscripts.

If the processor is not fast enough to keep up with the full frame rate,there needs to be a scheme to allow a random selection of frames to beprocessed. Since n laser on and in laser off frames are used for thebackground cancellation, while waiting to accumulate this number offrames, any excess laser on or laser off frames can be discarded.

Alliteratively a lock step synchronisation technique could be used sothat the computer was fed information about the state of the laser withrespect to the captured image. In any case, a minimum of one on frameand one off frame is required for the technique to work.

An alternative to the cancellation scheme described above is to simplysubtract laser on and laser off frames. Many on frames and off framescan be summed or averaged or low pass filtered, with the summing,averaging or filtering performed before and/or after the subtraction.

The result of the background cancellation is an image that ispredominantly composed of scattered light from the emitter, and someresidual background light and noise.

Frame Integration

At step 405 of FIG. 4 frame integration is performed. A number ofbackground cancelled frames are summed, averaged or otherwise low passfiltered to obtain a scattered light image with reduced noise. Byaveraging a number of frames, interference that is not correlated withthe laser on/off switching is reduced and the wanted (correlated)scattering information is retained. Typically the total number of framesused in the background cancellation and frame integration steps isapproximately 100 (i.e. approximately 3 seconds of video). Longerperiods of integration or lower filter cut-off frequencies may yield animproved signal to noise ratio, and allow a higher sensitivity system atthe expense of response time.

With reference to FIGS. 7a to 7c , the sequence of images shows theeffect of background cancellation and integration in the detection ofthe scattered light. The image intensity has been scaled to allow forbetter visibility to the eye. The particle obscuration level over theentire beam was approximately 0.15% per metre as measured by a VESDA™LaserPLUS™ detector, sold by the applicant. FIG. 7a shows the raw video,FIG. 7b highlights the region of integration, and FIG. 7c the scatteredlight in the presence of smoke after background cancellation andintegration.

Scatter Vs Radius Computation

At step 406 of FIG. 4 computation of the scatter as a function of theradius from the emitter is performed. Variations in intensity along thebeam due to system geometry and scattering may be remedied using thismethod. A data array is calculated containing scattered light levels inthe integration region versus radius, for example measured in pixels inthe captured image, from the laser source. Since a radius arc covers anumber of pixels inside the integration, the intensity of each pixelwithin a given radius interval is summed together. FIG. 8 is a graphicalrepresentation of how the integration region is segmented by arcscentred with respect to the emitter. In FIG. 8, triangle 80 representsthe expected integration area and the arcs represent different radiifrom the laser source. Each portion of the integration area lyingbetween a pair of arcs has its pixels summed and the sum is entered intothe scattered light data array. For pixels that are not clearly betweentwo of the arcs, rounding or truncation of the calculated radiuscorresponding to such pixels can be used to resolve the ambiguity.

Compute Geometry

At step 408 of FIG. 4, the geometry of the system elements/components isdetermined. Each pixel as described above (or image point) correspondsto a specific geometric configuration with respect to a scatteringvolume and the general case of such an image point is shown in FIG. 12.At each such point or pixel, the following parameters can therefore bedetermined:

-   -   θ—scattering angle.    -   r—the distance in meters from the laser source.    -   D—distance from camera to laser source.    -   L—physical length viewed by one pixel at a given point along the        beam.

A corrected intensity of pixels corresponding to a given radius, r, isthen determined for a real world system, in which the intensity ofpixels is multiplied by a predetermined scattering gain value, discussedbelow under Scattering Angle Correction, corresponding to the givenradius and a given scattering angle relative to a lossless isotropicscattering calculation. A resultant scattered data array is formed.

Scattering Angle Correction

A correction for scatter angle is logically determined in accordancewith step 409 of FIG. 4. As an input, the program requires a scatteringdata file, which contains for a given material, the scattering angle andits corresponding gain. The data in this file is generated by anempirical calibration process, and is intended to contain average valuesfor a variety of smoke types.

At each scattering angle as determined during the above geometrycomputation, the gain for every scattering angle is derived. The datafrom the input scattering data file is linearly interpolated so that forevery scattering angle an approximation of the forward gain can becalculated.

Compute Smoke Vs Radius

A determination of smoke for a given radius of the beam is performed atstep 407 of FIG. 4. To convert the scattered data array to smoke levelson a per pixel basis requires input of data D, d and θ_(i), as shown inFIG. 12. Any combination of lengths or angles that constrain thegeometry can also be used. D is the distance from the camera 82 to theemitter 84, θ_(i) is the angle made between the line from camera 82 andthe emitter 84 and the line corresponding to the path of the radiationfrom the emitter 84, and d is the length of the line perpendicular tothe emitted radiation that intersects the camera entrance pupil. Fromthis information, all other necessary information can be determined bytrigonometry and geometry. The geometry can be seen in FIG. 12.

For each element in the previously described Scatter vs Radius array,the values of L, θ and r, as shown in FIG. 12, are computed. L is thelength of the beam that is visible to one camera pixel.

Integrate Along Beam to Obtain Obscuration

At step 410 of FIG. 4, integration over beam image sectors is performedto obtain the detected obscuration. The beam length is divided into anumber of sectors to provide addressability along the beam. In order todistinguish between the laser source and scattering of the laser beam,the pixels around the laser spot location cannot be included as part ofa sector, as the intensity caused by scattering cannot be resolved,especially for an uncollimated source for which flaring may occurcausing residual intensity in the pixels surrounding the source.

Likewise at the camera end, due to the geometry of the set up, the fieldof view of the camera allows the beam to be viewed to within a fewmeters of the camera.

In order to provide a smooth transition between sector boundaries, asimple moving average filter is implemented. In fact, the beam isdivided into n+1 segments, and then a moving average is applied (oflength two segments) resulting in n sectors.

Each pixel along the beam captured image corresponds to a physicallength along the beam see FIGS. 8 and 12. This physical length getssmaller as the beam approaches the camera. So starting at the laser endand ignoring the pixels that are outside the end boundaries, theobscuration for a particular sector is the sum of all the pixelintensities after the application of the correction noted above, whichfall into the physical length and position as described by that sector.

For example, to determine the obscuration, 0, over the whole beam, givenas a sector size in pixel radius, r, as n to m,

$O = \frac{\sum\limits_{r = m}^{r = n}{{S(r)}{L(r)}}}{\sum\limits_{r = m}^{r - N}{L(r)}}$

where S is scattered light and L is given above.

As noted above, the beam length is divided into a number of segments todetermine individual smoke levels for each segment effectivelysimulating a number of point detectors. The output of these notionalpoint detectors can be provided to an addressable fire panel. This isbased on the theory that scattered light emitted from each segment ofthe emitted radiation will provide a different light output for a givenparticle density based upon the angle from the radiation path to thecamera and the number of pixels per segment. As the path of the emittedradiation comes closer to the camera that is as r increases in FIG. 12the angle θ_(r) increases. The number of pixels that contain scatteredlight will also increase due to the apparent widening of the beam in thedirection towards the camera 82. This increase in width is shown in FIG.8 and FIG. 13. FIG. 13 shows the emitted radiation from emitter 84. Theangle of the radiation spread is amplified for clarity. As the emittedradiation travels further from the emitter (that is as r increases), thenumber of pixels that coincide with the location of potential scatteredradiation increases. At the radius 86, close to the emitter, only twopixels are determined to be within the region of interest covered by thedetector, and the light from these pixels is summed and placed into anarray 90, being scattered light (r), which comprises a n times 1 arrayof information, where n is the number of pixels across the screen. Atradius 88, many more pixels are within the area of interest covered bythe detector, and they are all summed to obtain the amount of scatteringobtained within the covered region of interest. Calculated at array 92is the scattering radiation angle θ_(r), which will be different foreach pixel. That is, when r is small, θ_(r) will be small, and as rincreases, so does θ_(r). This information is important, as particles ofinterest in detecting certain events can have different scatteringcharacteristics. Very small particles (relative to the wavelength of theemitted radiation) scatter more uniformly regardless of θ_(r)(scattering angle), however larger particles scatter more in the forwarddirection, and reduce intensity as the angle Or increases. Quite oftenthe particles of interest, in this example smoke particles, arerelatively large particles and therefore it can be useful to employ atable of effective scaling factors of output of light for givenscattering angles θ_(r). Such tables are known in the use of smokedetectors using laser chambers to detect particles.

Array 94 contains the actual radius of the light captured by the pixels.Array 96 comprises the length of the segment of the emitted radiationencompassed by, in this case, one horizontal pixel in the captured imagein the frame of the camera. This information is used to ascertain thevolume of the emitted radiation and is used to assist in the calculationof the radiation intensity. Also, array 96 contains data on the smokeintensity at each point r, defined as smoke [r].

Alarm State

Finally with reference to FIG. 4, alarm states are calculated. The alarmstates for each sector are determined based on thresholds and delays anda priority encoding scheme, as per standard aspirated smoke detectors,or other parameters determined by the user.

The same method is used for the zone alarm level, except that final zoneoutput is the highest sector or the zone level, whichever is higher.

Fault Detection

The system may have provision for the detection of a fault condition,which is essentially the absence of the laser spot in the image. Thelaser on/off signal duty cycle may be checked to be within 33% to 66%over the number of frames used in one background cancellation cycle.

Alternative Embodiments

A number of alternative embodiments are available, depending onapplication and desired features. Unless otherwise specified, thegeneral principles of operation as described above apply to theimplementation of the following variations. For example, fault detectionmay be carried out in a number of ways.

In another application, the system described above could be used inapplications where measurement of obscuration was important, such asairports where fog may cause planes to divert if visibility falls belowa certain level. The system does not require ambient light to operate,and can therefore operate at night without additional lighting. Aninfrared camera could also be used with an infrared light source, wherethe light source, if of similar frequency to the detecting light, couldbe cycled so that the processor ignores frames illuminated for securitypurposes.

A typical security camera may take 25 images or frames per second. Smokedetection may only require detecting 1 frame per second or less.Therefore the remaining 30 images can be used for security purposes.

To give increased sensitivity, video processing software operatingwithin the detection sub-system (6,7) may be used to eliminate thecontribution of nuisance changes in video signals which are not in thelocation known to be occupied by the light beam. Software based systemswhich perform a similar function of processing distinct areas of a videoimage are known, for example in video-based security systems such asVision System's ADPRO™ products.

The emitter may be a laser, emitting polarised radiation. The laser mayemit visible radiation, infrared radiation or ultra violet radiation.Selection of the wavelength of the radiation may be dependent on thecharacteristics of the particles to be detected, as well as thecharacteristics of the apparatus and method to be employed in thedetection of the particles. Other types of radiation emitter maycomprise a xenon flash tube, other gas discharge tubes, or a laser diodeor light emitting diode. The light is preferably collimated to at leastsome degree, but if the optional area segregation using regions ofinterest is employed, a broader radiation beam may be emitted.

A further embodiment is shown in FIG. 11c , which employs two cameras102 and 104, and a single laser 106. In this embodiment, one camera canview the emitter, and the other the position or target where theradiation hits the wall 108. In such a configuration, it is desirable ifthe cameras 102, 104 are connected to the same processor or at leastcommunicate with each other. This system provides many advantages, suchas confirmation that the radiation is not blocked, and can be used todetermine more accurately a position of the emitter radiation withrespect to camera 104, which detects the forward scatter of light. Assuch, the degree of uncertainty of the position of the path of theemitted radiation is reduced, and the regions of interest can be reducedin size, increasing the sensitivity of the detector system. Further, asit is known that large particles, commonly caused by fire, forwardscatter more than smaller particles (often associated with dust), adetermination of particle characteristics can be made. If there issignificantly more forward scatter than back scatter for a particularsegment of the emitted radiation path, then it may be interpreted thatthe particle density at that particular segment consists of a proportionof large particles. It may be useful to compare this to other segmentsor other times, in order to ascertain characteristics of the event thatcaused the particles to be present in the first place.

While this invention has been described in connection with specificembodiments thereof, it will be understood that it is capable of furthermodification(s). This application is intended to cover any variationsuses or adaptations of the invention following in general, theprinciples of the invention and comprising such departures from thepresent disclosure as come within known or customary practice within theart to which the invention pertains and as may be applied to theessential features hereinbefore set forth.

As the present invention may be embodied in several forms withoutdeparting from the spirit of the essential characteristics of theinvention, it should be understood that the above described embodimentsare not to limit the present invention unless otherwise specified, butrather should be construed broadly within the spirit and scope of theinvention as defined in the appended claims. Various modifications andequivalent arrangements are intended to be included within the spiritand scope of the invention and appended claims. Therefore, the specificembodiments are to be understood to be illustrative of the many ways inwhich the principles of the present invention may be practiced. In thefollowing claims, means-plus-function clauses are intended to coverstructures as performing the defined function and not only structuralequivalents, but also equivalent structures. For example, although anail and a screw may not be structural equivalents in that a nailemploys a cylindrical surface to secure wooden parts together, whereas ascrew employs a helical surface to secure wooden parts together, in theenvironment of fastening wooden parts, a nail and a screw are equivalentstructures.

“Comprises/comprising” when used in this specification is taken tospecify the presence of stated features, integers, steps or componentsbut does not preclude the presence or addition of one or more otherfeatures, integers, steps, components or groups thereof.

What is claimed is:
 1. A method of detecting smoke within a monitoredregion: directing a beam of radiation across the monitored region;capturing images of monitored region, including at least part of a pathof the beam of radiation through the region; analizing the capturedimages to determine the presence of smoke on the basis of an increase inlight scattered from the beam; integrating intensity values from theimages; determining individual smoke levels for a respective pluralityof portions of the monitored region using the integrated intensityvalues and different thresholds for each portion, each of the smokelevels being based on an increase in light scattered from a portion ofthe beam within a corresponding portion of the monitored region.
 2. Themethod of claim
 1. wherein the method includes simulating a plurality ofpoint detectors corresponding to different segments of the beam.
 3. Amethod as claimed in claim 1 wherein the method includes, outputting asmoke level to an addressable fire panel.
 4. A method as claimed in 2wherein the method includes, outputting a smoke level corresponding to aplurality of point detectors to an addressable fire panel.
 5. A methodas claimed in claim 1, wherein the method includes dividing each imageinto subregions corresponding to said portions of the monitored region,each subregion including a corresponding portion of the path of thebeam.
 6. A method as claimed in claim 1 which further comprisesadjusting a level of light received by the image capture device for eachpixel by a predetermined scattering gain corresponding to a radius fromthe light source and scattering angle, θ.
 7. A method as claimed inclaim 1 which further includes: integrating over a plurality pixelscorresponding to the portion of the monitored region to obtain adetected obscuration for the portion of the monitored region.
 8. Amethod as claimed in claim 1 which includes determining an integrationregion that contains the path of radiation.
 9. A method as claimed inclaim 8 which further includes: performing background cancellation ofthe images of the integration region.
 10. A method as claimed in claim 9wherein the beam of radiation is modulated, and background cancellationincludes summing n “on” frames and m “off” frames.
 11. A method asclaimed in claim 1 which includes modulating the beam of radiation andlow-pass filtering a plurality of captured images to remove interferencethat is not correlated with the beam modulation and retain lightscattering information.
 12. A smoke detection apparatus for monitoring aregion, comprising: an emitter for directing a beam of radiationcomprising at least one predetermined characteristic into the region; animage capture device arranged to capture a plurality of images of themonitored region; a processor for analyzing the plurality of imagesaccording to a method of claim 1.